The use of aluminum in heat exchangers is now widespread in applications such as automotive, off road equipment and heating ventilation and air conditioning (HVAC) systems. Extruded tubing is often used due to the ability to produce complex thin wall geometries such as mini microport (MMP) tubing which improves heat transfer. Such tubes are typically connected to fins and headers/manifolds to create the heat exchanger using controlled atmosphere brazing (CAB). Resistance to failure by pitting corrosion is an important property of these units which can be subjected to corrosive environments such as road salt, coastal environments and industrial pollutants. At the same time, the expectations in terms of lifetimes of the units and customer warranties are increasing and there is a continuing need to improve the corrosion performance of such systems. The extruded tubing is typically the thinnest walled component of such heat exchangers and the most likely to fail by corrosion first. Often the tubes are zincated either by thermal arc spray or by roll coating with a zinc containing flux which adds a measure of sacrificial corrosion protection. However, the inherent corrosion resistance of the underlying tube material remains a key component of the protection mechanism, particularly when the sacrificial Zn rich layer has been removed by corrosion.
A number of “long-life alloys” have been developed in an attempt to address this problem. U.S. Pat. No. 6,939,417 describes controlling the levels of Cu and Ni when using AA3000 and AA1000 series aluminum alloys to improve corrosion resistance. This patent is incorporated by reference herein in its entirety and made part hereof.
U.S. Pat. No. 5,286,316 provides an essentially copper free aluminum based alloy composition useful in automotive applications, in particular, heat exchanger tubing and finstock.
U.S. Pat. No. 6,638,376 relates to an aluminum alloy piping material exhibiting good corrosion resistance and having an excellent workability, such as bulge formation capability at the pipe ends.
U.S. Pat. No. 7,781,071 relates to extruded tubes for heat exchangers having improved corrosion resistance when used alone and when part of a brazed heat exchanger assembly with compatible finstock. This patent is incorporated by reference herein in its entirety and made part hereof.
U.S. Pat. No. 8,025,748 teaches an extrudable aluminum alloy ingot with 0.90-1.30Mn, 0.05-0.25Fe, 0.05-0.25 Si, 0.01-0.02Ti, less than 0.01Cu, less than 0.01Ni and less than 0.05 magnesium, with the aluminum alloy billet homogenized at a temperature ranging between 550 and 600° C. This product has been successful commercially, but further improvements in corrosion resistance are required for the demanding HVAC market. At the same time, availability of primary aluminum with low Ni content is decreasing globally causing a general degradation of pitting corrosion resistance.
The present composition and method are provided to address the problems discussed above and other problems, and to provide advantages and aspects not provided by prior compositions and methods of this type. A full discussion of the features and advantages of the present invention is deferred to the following detailed description, which proceeds with reference to the accompanying drawings.